The general advice is that after resistance training you should consume plenty of simple carbs to spike your insulin level and thus help getting nutrients to your muscles (and to replenish your glycogen stores).

However, insulin is also responsible for storing excess calories as body fat which is clearly a bad thing and should be avoided as much as you can even when you're bulking. Recent research also indicates that you don't need to skyrocket your insulin level since it gives no extra benefit, so a low-GI food with protein (e.g. whey shake and some fruit) should also do the job.
E.g.: Alan Aragon's Research Review

2 Answers
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Carbs post workout are a good thing. If you've exerted yourself you have depleted all your quick reserves and you need to replenish them. However simple carbs (like a candy bar, sugar, high fructose corn syrup, etc) are never a good idea unless you are dangerously hypoglycemic--a condition that diabetics have to deal with occasionally. The processed carbs will cause your body to go hyper for a bit and then crash badly afterwards.

+1. I often also heard the advice: If you really must eat candy, then do it after your workout. Not because it is good thing, but because at this time it does the least damage.
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Daniel RikowskiJul 7 '11 at 13:26

I'm trying to find out whether simple carbs are really better PWO for muscle building (via creating an anabolic environment) as most body builders advocate, or if I should rather take complex carbs e.g. oatmeal, some fruit, rather take enough slow carbs before the workout or just skip the PWO carbs if I'm cutting. So the goal is to start muscle building (or at least spare muscle) PWO, without gaining fat from simple carbs.
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ldxJul 7 '11 at 17:03

Unless you have a peer reviewed study that proves this, I would be wary of such claims. I honestly don't think there are any shortcuts. As to not gaining fat, as long as the glycemic load is low enough (i.e. amount times G.I. as percentage) you should be OK. If you aim for a total G.L. lower than 90 for the day all will be fine. The problem is finding the G.I. information for foods as this is very difficult to test and there are no easy correlations between ingredients and G.I..
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Berin LoritschJul 7 '11 at 17:31

The majority of the fuel you will be burning for your resistance workout will be glycogen (=stored sugar).

Muscle glycogen (glucose = dextrose) will be a large chunk of that (say 100 g from an intense session), while liver glycogen (fructose) will comprise maybe 1/5 of that.

An ideal PWO meal should be easily digested/ broken down and ready to get where it needs to go.

This means no fiber. no fats. Whey protein is best here. And simple sugars - mainly dextrose ( next best thing: starches) and a little fructose (banana).

A good recommendation here is 2:1 sugars to protein. So say 50 g dextrose/fructose, 25 whey.

Without going into too much science here, your body is primed to replace what you have spent after a workout. Cells are very permeable/sensitive and it will only take a little insulin to usher the aminos and sugars into them. After a workout is the absolutely hardest time "to get fat".

If you decide to forgo post-wo carbs - you will 1) be sore for a longer period of time (depleted glycogen doesn't feel too good) and 2) Your next workout will be hurt (because fuel stores will be way down).